Electric valve converting apparatus



Au 10, 1937. AUGER 2,089,872

ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1935 9 Sheets-Sheet l I ll II" "II "II Invent or: Jean Auger,

His Attorney.

Aug. 10, 1937. AUGlER 2,089,872

ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1935 9 Sheets-Sheet 2 Inventor: Jean Ajgier, b 6? M ttorney.

Aug. 10, 1937. J. AUGIER ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1935 9 Sheets-Sheet 3 lnven fior" Jearw A i r, by 7/ fii is Attorney Aug. 1 0, 1937. J AUG|ER 2,089,872

ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1935 9 Sheets-Sheet 4 Pi .5. /14 2Z3 Z2 2/ nu nu |r|| nu Inventor: Jean Au ier:

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Aug. 10, 1931. A l R 2,089,872

ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1955 9 Sheets-Sheet 5 zoa | I /20d'd 2/Jd: I

l V 205 l l I l Inventor: Jean Au Ker,

H is Attorney Aug. 10, 1937.

J. AUGIER v ELECTRIC VALVE CONVERTING A PPARATUS Filed June 28, 1935 9 Sheets-Sheet 6 Inventor Jean Auger,

by M

ttorney.

Aug. 10, 1937. AUGER 2,089,872

ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1935 9 Sheets-Sheet 7 Fig.7.

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305K aosy 3/51 3/5.Y 3257 325')! Inventor:

Jean Au ier,

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Aug. 10, 1937.

.J. AUGIER ELECTRIC VALVE CONVERTING APPARATUS Filed June 28, 1955 9 Sheets-Sheet 8 Fig.9.

Jean Au y 79/0 afi ijj H i s Attorney.

Aug. 10, 1937. J, AU ER 2,089,872

ECTR IC VALVE CONVERT ING APPARATUS Filed June 28, 1935 9 Sheets-Sheet 9 Fig. 10.

Inventor: Jean Auger",

Attorney.

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w w W m m VVYWWVWVWVW Patented Aug. 10, 1937 UNITED STATES PATENT OFFICE Jean Augier, Belfort, France, assignor to General Electric Company, a corporation of New York Application June 28, 1935, Serial No. 28,961 In France July 4, 1934 Claims. (Cl. 175-363) My invention relates to electric valve converting apparatus and more particularly to a method of operation of such converters over a wide range of output.

5 It is known that one can regulate the direction and the magnitude of the voltage of electric converters involving a discharge in ionized gases or vapors by controlling the instant of ignition of the various anodes of the rectifier, said instant being.capable of a variation which can be obtained by utilizing either the well-known properties of the control grids or by starting the cathode spot at the proper moments, or by any other means.

Up to the present time, it has been customary to retard by the same electric angle, the ignition of all the different anodes with respect to the moment of the cycle at which the anodes would naturally start. It is known that if this time lag is kept constant, the direction and the magnitude of the continuous potential at the terminals of the converter depends on the value of that time lag. The converter can then permit an exchange of energy between a direct-current network and an alternating-current network. If, however, the time lag in the anode ignition is periodically variable, the converter will furnish an undulating (rippled) voltage, and that property makes it possible to interconnect two alternating-current networks having different frequencies.

The converters which have been built according to these well-known arrangements operate reliably and accurately, but they have, nevertheless, some shortcomings.

On the one' hand, the apparent or reactive power on the a-c side of the converter, in the case of a rectifier or inverter, does not depend upon the real power utilized, but solely on the 40 current density on the d-c side. The result is that the power factor is nearly proportional to the ratio between the continuous voltage obtaining in service and the maximum voltage. If the continuous voltage is to be regulated. within rather wide limits, the power factor is considerably lowered when one departs from that maximum value, and the consumption of reactive energy becomes enormous. Similar phenomena manifest themselves in the case of apparatus which operate as frequency changers or modulating converters.

On the other hand, the amplitude of the harmonies of the rectified voltage increases very 55 rapidly with the regulating range, and for that reason, the filters which are frequently necessary become often very expensive.

These two disadvantages have greatly handicapped the development of electric converting 60 apparatusinvolving a discharge in ionized gases or vapors, and particularly the development of mercury-vapor converters.

These disadvantages could be reduced to a certain degree by adopting a method of stepby-step, or rather stage-by-stage regulation, with the stages more or less far apart, in which the transformation ratio of the transformer connected to the anodes of the converter would be varied, and by completing the regulation by influencing, within the intervals between the stages, the moment of ignition of the anodes. This method has the disadvantage that it introduces the complication of necessitating a variable-ratio transformer.

It is an object of the present invention to provide a new method of operation of electric valve converters which permits improving, to a much higher degree, the power factor and the shape of the voltages and currents in converters and other electrical apparatus involving a discharge in ionized gases and vapors, without necessitating the application of variable ratio transformers.

This new method is mainly characterized by the fact that while the various members which constitute the converter or similar apparatus are so arranged that several anodes can be made to deliver current simultaneously, and while several possible relations exist between the phases of the alternating anode potential which can furnish current simultaneously, one brings about, depending upon the regulation to be established, the operation of the converter or other apparatus according to one or the other of these phase relations, and change-over at appropriate moments from the operation according to one of these phase relations to the operation according to another of these relations, said change-over being attainable through any appropriate means, but preferably, according to the invention, by a suitable control of the starting of the anodes (control electrodes, starting the cathode spot at the suitable instants, etc).

It is known that when several anodes of the same or of several converters or of other apparatus involving a discharge in ionized gases or vapors are coupled to each other by means of intermediate absorption coils, inter-phase transformers, current dividers, etc, the continuous potential at the terminals of the system is at any instant the same as if all the anodes thus coupled which supply power simultaneously were fed by one source the instantaneous voltage of which would be equal to the arithmetic mean of the values of the voltages applied to said anodes, whether these voltages are in phase or not. If one modifies the relation existing between the phases from the voltage sources supplying the anodes which have a simultaneous output, the operation is similar to that which would occur if one were to modify the value of a common Any rectifying, inverting, modulating, or frequency-changing apparatus, etc. to which this regulating method is applied will be able to operate with a theoretical phase displacement factor which is equal to unity, (the reactive losses due to the transformers being disregarded), for certain values of the rectified, inverted, modulated or frequency-changed voltage, etc., which values correspond to those phase relations for which the conditions necessary to the commutation between anodes are established at the suitable instants.

In order that the commutation between anodes be possible, the potential of the anode that must start with respect to the cathode must be at least equal to the anode-cathode potential of that anode that is to be extinguished.

For certain phase relations between the anodes that operate simultaneously, these conditions of commutation may be accomplished at the moments where the resultant voltage curves that intersect each other are defined by the phase relations under consideration and that correspond respectively to the anodes which deliver power before and after the commutations between anodes. The theoretical phase displacement factor will be then equal to unity, if one permits the commutation between anodes at the instant which has been just considered, and the value of the voltage of the converter or any other apparatus will be proportional to the geometrical sum of the vectors of the voltages applied to the anodes that function simultaneously.

In the case of other phase relations of the voltages applied to the anodes that function simultaneously, it may occur that the commutation between anodes is only possible after the instant as defined above. In the case of such phase relations, the theoretical phase displacement factor will be less than unity. It will, however, exceed that which would be obtained by simply retarding in the well-known manner the instant of starting the anodes by the value of the electric angle that is necessary to obtain the same value of the converter voltage or other apparatus underconsideration.

In the preceding explanations we have assumed that the anodes that were functioning simultaneously were connected in parallel. However,

the invention obviously may be applied to series and series-parallel connections.

In the case where the anodes that operate simultaneously are connected in series, the voltage at the terminals of the converter or other apparatus is at each instant equal to (disregarding the voltage drops) the arithmetical sum of the voltages applied to the diverse anodes. As in the preceding cases, there will exist a certain number of possible relations between the phases of the voltages applied to the anodes operating simultaneously, for which relations, the commutation conditions being established at the suitable instants, the theoretical phase displacement factor will be equal to unity, the voltages attained (disregarding the voltage drops) being proportional to the geometrical sum of the voltage vectors corresponding to each phase relation under consideration.

In the case of all the voltages from the converter or from any other apparatus which are created by obtaining, through suitable phase relations between the voltage sources feeding the anodes that operate simultaneously, a phase displacement factor equal to unity, it is quite evident that the undulation or ripple factor is the same as in the case of the maximum voltage and consequently, is much lower than if the regulation had been brought about simply according to the well-known method of retarding the ignition of all the anodes. Thus, by virtue of the method according to the invention, the shape of the voltages and currents of the apparatus to which this method is applied is improved.

Inasmuch as the number of phase relations which can be obtained is limited, the regulation which is realized directly in this manner will be a regulation by stages. To obtain a progressive, gradual regulation and to realize intermediate values between these stages or levels, one can utilize any suitable means of regulation, but particularlyand these special means must be considered as constituting in themselves a part of the invention-one may utilize the method which consists in varying the moment of igniting the anodes, or the method consisting in causing to alternate, according to a suitable cadence, operating stages corresponding to several different phase relations between the voltage sources that supply.

the anodes which deliver energy simultaneously.

It is quite obvious that the use of the first one of these special methods of intermediate regulation will have the efiect of reducing between the various levels the value of the theoretical phase displacement factor. But, since this regulation will take place only between two rather close values of the voltage of the converter or other apparatus, for which the theoretical phase displacement factor is equal to unity or approaches that value, one will obtain, even for intermediate values, an improvement of that phase displacement factor. In fact, the latter is equal to the ratio between the operating voltage or intermediate voltage and the maximum voltage. highest voltage which was obtained by the coupling of the anodes of the level under consideration, or which would be obtained by assuming that the commutation of the anodes functioning in succession is possible at the moments which would correspond to unity for the theoretical phase displacement factor. Even better results can be attained by using the second of the special means cited, that is to say by causing to alternate at a suitable cadence, whenever possible, the operating condition of the converter or other apparatus according to various phase relations between the sources supplying the anodes that function simultaneously. In general, in order to get a given operating voltage between two voltages which would prevail, respectively, with two different phase relations, it will be interesting to cause the alternate functioning of the converter or other apparatus according to one or the other of these phase relations.

It will be also advantageous, in order to improve the undulation or ripple factor of the voltage, to have the operation alternateaccording to the various phase relations, generally two, at a cadence which is as rapid as possible, that is,

practically at the frequency of the commutation between anodes.

Finally, one may note that for certain intermediate regulations, one can obtain a theoretical phase displacement factor equal to unity, by suit- This maximum voltage is here only the I ably selecting a half-wave frequency for the operating state from the limit phase relations, in general by selecting a frequency which is lower than that required by the lowest undulation ratio. This frequency may be selected according to the requirements.

At any rate, it will be advantageous to see to it-and that can be easily donethat the mean load on the various anodes will remain the same for all the operating voltages.

The novel features which I believe to be characteristicof my invention are set forthwith particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in connection with the accompanying drawings in which Figures 1, 1a, 4, 5, 5, 9, and 10 disclose diagrammatically circuit arrangements to which my invention is applicable while Figures 2, 3, 6, '7, and 11 are graphic representations for explanatory purposes.

Referring to Fig. 1, which represents a physical embodiment of the invention applied to a twelveanode mercury-vapor converter, the alternating current network I and the direct-current network 2 are interconnected by means of a transformer 3 provided with a primary winding 4 connected to the network i and with two identical six-phase secondary windings 5 and I5 comprising, respectively, six phase windings 5a-5b-5c5d-5e' 5r and |5al 5b--|5c 5s|5e! 5r enumerated in the order of the direction of rotation of the phases. These windings are respectively connected to the anodes 6aEb6c-iis6e-6r, I ialtbltc-itd lfielfir of a mercury-vapor rectiller it provided with a cathode I I and with control electrodes or grids, such as l2. The control circuits for the grids or electrodes which are not shown may be any of the known arrangements as will be apparent to those skilled in the art. The operation of the control circuits of these grids, depending upon the rules which will be set forth later on and conforming with the invention, may be accomplished according to any appropriate means. The neutral points 1 and I? of the two secondary windings 5 and I5 are interconnested by of an absorption coil or interphase coil ii, the center point 13 of which is connected to one side of the line 2, the other side of the line 2 being connected to cathode II. The converter comprising the transformer 3 and the rectifier i8 is resolved, according to the invention, into two elementary groups that operate simultaneously, one of which comprises the secondary winding 5 and the anodes 6a, Eb, 6c, 5e, So, (if, while the other comprises the secondary winding it and the anodes its, I51), I60, lea, IE9, if.

The diagrams represented in Fig. 2 will facilitate the understanding of the invention.

In Fig. 2 the curves a, b, c, d, e, 1 represent, respectively, as a function of the time plotted on the abscissa, the voltage waves that are induced, respectively in the secondary windings (5a, l5a) (5b, i513) (5c, '50) (5d, l5d) (5e, I56) (5r, Hi).

If the control voltages that are applied to the control grids l2 are such that each anode may start and supply energy as soon as its potential becomes equal to or exceeds that of the anodes that supplied energy just previously, the anodes ia and its for instance, which had started at the time 151, will extinguish at the time t2 (assuming the leakage losses in. the transformer 3 to be zero), when the anodes 6b and Hit start.

The diagram of the rectified voltage is represented by the upper rippled curve ABCDEFG. The rectified voltage is then a maximum, and the theoretical phase displacement factor is equal to unity. Each anode supplies, for one-sixth of a cycle, a current equal to one-half of the rectified current, since two anodes operate in parallel.

If one prevents, through the action of the control grids, the starting of the anodes I6a, I60, l6e, on the one hand, and of 6b, 6d, 6; on the other hand, and if the starting of the other anodes is permitted as soon as their potential becomes equal to or exceeds that of the cathode (the inverter operatiomwill give similar results), the operation will be the same as the well-known operation of a six-phase rectifier with an interphase coil. That is to say, the anode lBb starts at the time t3 and becomes extinguished at the time it; when anode [6d starts in turn, anode 60 starting at the time is when anode 6a becomes extinguished. During the simultaneous energy output of two anodes, 6a and IE1) for instance, their potential with respect to the center point 53 of interphase coil 8 is, at each instant, equal to the mean of the voltages induced in windings 5a and 15b to which the corresponding anodes are connected, the difference between these two voltages being absorbed in coil 8. The curves fa, at), be, ed, do and ef represent, respectively, the values of the mean voltage between the voltages induced in the phase windings 5a and |5b (or 4 5a and 5b), 15b and 5c (or 5b and '50), 5c and |5d (or 5c and 5d), etc.

The rectifi d voltage is then represented by the rippled curve AiABiBClCDiDEiEFiFGiG The ratio between the value of the rectified voltage obtained in this manner and the value of the maximum rectified voltage Em is equal to and the theoretical phase displacement factor is still equal to unity.

The rectified voltage is still shared by two anodes which supply energy simultaneously. The maximum value of the instantaneous anode current has not increased, but on the other hand, only six anodes supply energy, each one during one-third of a cycle, the three others remaining inactive. One can avoid increasing the dimensions of the anodes and of the transformer and one can retain the full utilization of the rectifier by changing over from time to time from the operating anodes to those that are at rest, in other words, after having the anodes supply current, anodes 6b-6d6f-|6a-|6ci6e remaining inactive, one can release the anodes fib6d6rl6a|6cl6e and block the anodes 6a-6c-6el6bl6d-l6i which are inactive in their turn. If these permutations are made (for instance automatically) at equal time intervals that are relatively short with respect to the ther mal time constant of the anodes, the mean charge of the anodes is the same as in the case of maximum-Voltage operation and the utilization of the rectifier is not decreased.

In the mode of operation which has just been studied, two anodes (to which voltages are applied that are sixty electric degrees out of phase), are allowed to supply energy simultaneously. A new value of the voltage may be obtained by allowing only two anodes to supply energy, to which anode-s voltages are applied which are one 15 point 04.

hundred and twenty electric degrees out of phase.

If, for instance, in the time interval between is and t7, the anodes 6b and [6d supply energy simultaneously, the other anodes being blocked "5 by their control grids, the potential of these two anodes with respect to point [3 is represented by the curve bd, which is the mean of the two voltage curves b and d. If at the time it which slightly precedes time is, when the voltage curves T 0 and d intersect each other, the anode '60 is released, that anode will extinguish anode lfid, which is less positive and it will assume its load; .the potential which anodes 6b and I60 have in common, follows the curves be starting from the If at the time ts, the anode Go is liberated, it starts and assumes the load of anode b which becomes less positive. The current flow is then insured by the two anodes I60 and 6e whose common voltage with respect to point l3 is represented by the curve ce.

If the leakages of transformer 3 were zero, the commutations would be instantaneous; times such as $7 and is for instance,,could be very close to each other and times such as C5 and E3 would practically coincide. The phase displacement factor would still be equal to unity in the case of a rectified voltage equal to one-half the maximum voltage.

As in the preceding case, it will be advanta- 35 geous to change, at suiiiciently short intervals,

the groups of active and inactive anodes, in order to preserve a good utilization of the rectifier of the transformer.

If one wishes to obtain an intermediate voltage between the maximum voltage with a value Em and a voltage with a value i45 it may suffice to retard as already known, the

moments of simultaneous ignition of the anodes 6a and lea, 611 and b the diagram of the rectified voltage is then represented by the rippled curve AeAvBBsBvCCeCvD. The rectified voltage 50 and the phase displacement factor vary then as the cosine of the angle of lag at the starting.

In Fig. 3, the straight line KZ]. represents the variation of the theoretical phase displacement factor as a function of the ratio between the 55 service voltage and the maximum voltage Em,

that ratio being plotted as abscissas.

Likewise, one can obtain voltages below the value by retarding the starting of the anodes in the second modus operandi that has been described.

65 The diagram of the rectified voltage is then represented by the rippled curve In Fig. 3, the straight line KZz represents, for that 7 operatien, the variation of the phase displacement factor as a function of the rectified voltage. The same principle of regulation by retarding the ignition of the arc cannot be applied in order to lower the service voltage corresponding 75 to the third modus operandi which has been described. It will be noted that, in order that the diagram of the rectified voltage may pass from curve bd to curve ce, it is necessary that the commutation between the anodes '60 and its take place prior to the time ta. Beyond the time is, the voltage of the anode I60 drops below that of the anode His, and the transfer of the load from anode l6d to anode I60 is no longer possible.

In the following it will be explained how it is possible to adjust the rectified voltage for values which are less than one-half the maximum voltage. In the method of regulation which has just been described, the theoretical phase displacement factor is represented as a function of the rectified voltage (Fig. 3) by the rippled line Z1P1Z2P2Z3. It will be seen that it is always equal to or that it exceeds the phase displacement factor represented by the straight line KZI, which will be obtained by regulating the voltage by simply retarding the starting of the anodes.

A substantial amelioration of the phase dis placement factor can be obtained by alternating, as was mentioned above, the operation of the converter according to two phase relations. If it is desired that the ripple of the rectified voltage become a minimum, the two operations must be alternated according to the commutation cadence of the anodes. If it is assumed, for instance, that at the time is, the anodes 6e and [6e supply energy up to the time im, the diagram of the rectified voltage is represented in Fig. 2 by the arc ElOF. At the moment he, only the anode (if, for instance, is released, anode if being blocked by its control grid. The anode 6r assumes theload of anode 6e, and the two anodes We and 6: supply energy simultaneously, the diagram of the rectified voltage follows the curve cf. If at the time tn the anode lfif is released, it starts and extinguishes the anode H68, with the result that the diagram of the rectified voltage follows the curve I. It will be seen that it is possible to obtain a voltage diagram such as is represented by the rippled curve EloFFllFloG-AllAlo. When the time intervals such as t1ot11 increase, while they remain at the same time below one-twelfth of a cycle (fro-7512), the rectified voltage and the phase displacement factor drop simultaneously. However, the phase displacement factor decreases less rapidly than the rectified voltage. In Fig. 3 the curve Z1M1 shows the variation of the theoretical phase displacement factor as a function of the rectified voltage. When the moment in coincides zvith the moment in, the rectified voltage is equal and the theoretical phase displacement factor passes through a minimum the value of which is close to 0.965.

Values of rectified voltages between of anode [6e (curve fa). At the moment tn, the anode I6: is released and assumes the load of anode 6a which becomes extinguished (curve 1) the same phenomenon taking place at each sixth of a cycle.

When the time interval 1z-t1s increases, the rectified voltage drops and the phase-displacement factor improves. At the limit, when the time interval fin-in is equal to one-twelfth of a cycle, the second modus operandi which has been studied is encountered, the rectified voltage is equal to and the phase-displacement factor is equal to unity. In Fig. 3, the segment MIZ2 shows the variation of the theoretical phase displacement factor.

For voltages that are slightly below 3 IE and 2 can be obtained with a better phase-displacement factor. In the preceding it has been explained that in order to obtain the change-over from the simultaneous output of anodes 6s and 16d (curve bd) to the simultaneous output of the anodes I60 and 6e (curve cc), it is necessary to go through the simultaneous output of anodes 6b and '60 (curve be). A voltage value comprised between the values %E and -E can be obtained by changing over from the output of anodes 6b and id to the output of anodes 6b and [60, no longer at the time 157 which is as close as possible to the time is, but at a time 14 which lies considerably ahead. In this manner one can obtain the diagram of the rectified voltage D3C14Cl5E3Dl4Dl5F3. In Fig. 3 the segment M223 shows the variation of the theoretical phasedisplacement factor as a function of the rectified voltage, the phase displacement factor returning 0 gradually from the minimum value of for the rectified voltage to unity for the voltage time t16. The voltage diagram follows then the curve ef to the point B16, corresponding to the time in where the starting of the anode 6b alone is permitted. The voltage diagram follows then the rippled curve F5F4B3BlfiBl'7A5A4C3C16Cl7. The phase-displacement factor and the rectified voltage decrease simultaneously. In Fig. 3, the curve KM3Z3 shows the trend of the variation of the phase-displacement factor.

There still exists a fourth possible phase relation which corresponds to a theoretical phasedisplacement factor equal to unity and to a rectified voltage which is theoretically zero. To this end, it would sufiice to have two anodes supply energy permanently, to which voltages are applied which are in phase opposition, for instance the anodes 8c and Hit (the consumption in reactive power would be then zero). It will be easily seen that it is possible to obtain also voltages that are not equal to zero.

With anodes 6c and [6f supplying power simultaneously, and the instantaneous rectified electromotive force being zero, one can, at the time t18, for instance, release anode I6d which assumes the load of anode if. The rectified-voltage curve will then assume the shape of the arc D18A19 of curve cd. At the moment in; (whose representative point on the time axis coincides with the point A19, whose ordinate is zero), the anode 6a is released, and inasmuch as the voltage which is applied to it becomes equal at that moment to that of anode 6c, anode 62. starts and causes anode fie to become extinguished. Anodes 6a and 6d to which voltages are applied in phase opposition, deliver their energy simultaneously, and the instantaneous rectified voltage is zero. One can thus represent the rectified voltage by means of a broken line C20C18F19D20Dl8A19E19E18 the point D20, the ordinate of which is zero, coinciding on the time axis with the representative point of time in. As in the preceding cases, the changes will be made so that all the anodes will be equally loaded.

In Fig. 3, the curve M4M3 shows the trend of the variation of the phase-displacement factor.

For the rectified voltages that are comprised between zero and the maximum voltage Em, the theoretical phase-displacement factor is represented by the rippled curve M4M3Z3M2Z2M1Z1. With the method of regulation consisting simply in varying the instant of starting the anodes, the phase displacement factor will be represented by the straight line KZI. It will be seen that the application of the invention makes it possible to considerably decrease the reactive power consumption.

If a slight increase in the rippling factor of the voltage is permissible, the phase displacement factor may be still further increased. For instance, the rectified voltage having a value may be obtained with a phase displacement facphase inductor 26.

tor equal to unity by having the operation follow the rectified-voltage diagrams similar to that represented by the rippled curve DsDFsABGCs.

The foregoing descriptions relate to the operation as a rectifier, but it is quite evident that the converter shown in Fig. 1 could also operate as an inverter. The rippled curve MNOPQRSTUVXYZ of Fig. 2 represents theoperating diagram corresponding to a continuous supply voltage comprised between the values Em and The foregoing explanation regarding the rectifier operation will enable any person acquainted with the art to understand the inverter operation.

In Fig. 1a there is disclosed a portion of the grid control circuit by means of which the control grids of a l2-anode tank rectifier may be controlled in accordance with the modes of operation described above. While for purposes of clarity only a portion of the grid control circuit is shown as applied to the circuit arrangement disclosed in Fig. 1, it will be apparent to those skilled in the art that similar grid control circuits may be applied to the 12-anode rectifier arrangements disclosed in the other figures of this application.

A suitable phase shifting device 20 energized from the alternating current line i is arranged to be connected by means of either of the switches 2 i, 22, to the primary windings of the transformers 23, 2A1 and 25. These transformers whichare preferably of the type which produce peaked impulses in the secondary windings thereof are provided with primary windings having their midpoints connected together through the three The secondary windings of these transformers 23, 2e and 25 are arranged to be connected by means of switches 2'5, 28 and 29 to the primary windings of the transformers 3i),

3i and 32. In order to prevent excessive current from flowing through the transformer windings, current limiting resistors 33, 34 and 35 are connected respectively between the windings of the transformers 23, 2t and 25 and the windings of the transformers 33, 3| and 32. The primary windings of the transformers 30, iii and 32 may also be provided with shunt circuits including rectifiers 3S, 3? and 38 which may be of the dry contact or copper oxide type for the purpose of preventing the negative peak from being transferred to the secondary windings of the transformers 36, 3E and 32. The secondary windings of the transformers 3B, 31 and 32 are connected, respectively, to the grids of the anodes 6a, 6c and 6c. The grid circuits of these anodes are completed by means of suitable slip rings through the alternating current generator 39 and the direct current generator 40 to the cathode H of the tank rectifier it. The alternating current generator 39 is excited by means of a direct cur rent field M, which maybe energized from the direct current circuit 42 by means of a switch 43. The direct current generator it is excited by means of the field 44 which is connected to the direct current circuit 42 by means of the switch 35. Both the direct current generator 40 and the alternating current generator 39 are drivenby means of a synchronous alternating current motor it which may be energized from any suitable source of alternating current such as the alternating current line i.

By means of a similar arrangement of circuit elements controlled by the switches ll to 52these circuit elements shown to the right in Fig. 1a op-- crate to control the grids for the anodes 16b, His and lfir. Similar sets of apparatus are provided for each of the groups of grids for the anodes 6b, 6d 6f, andlfia, 6c, is.

For full load operation the phase shifters 2'8, 54 and those of the other two groups of control grids are set so as to permit each anode to start as soon as its potential becomes equal to or exceeds that of the anodes which supplied energy just previously. For this operation, the switches in all the apparatus corresponding to switches 2! to 41, 27 to 29 and 49 to i are in the closed position,

For operation where the maximum rectified Voltage and the theoretical phase displacement factor is still equal to unity the starting of the anodes 6b, 6d, 6i and 6a, H60, Hie is prevented. This is accomplished by opening the switches 27 to 29 and 49 to 5! and then opening the switches 2i and 41 to remove the positive voltage peaks applied by the grid circuit transformers. In order to prevent these grids from starting a negative bias is applied to the grids by closing the switches 45 and 53 which control the energization of the field coils of the direct current generators of the respective grid circuits.

For operation which will produce a rectified voltage equal to one-half the maximum voltage and a phase di placement factor equal to unity, only two anodes supply energy at any time. The voltages applied to these anodes are 120 electrical degrees out of phase. For this operation all of the grid circuits are provided with a negative biasing voltage obtained by closing the switches to the field coils of the various direct current generators. The grids of the groups of anodes Be, Be and 6e are energized so that the anodes become conductive 30 electrical degrees earlier than they would under full load condition. This is accomplished by closing switches 27, 28 and 29 and also 2! and adjusting the phase shifter 26 so as to advance the phase to the proper point. The grids of the anodes its, its, lfir are also energized with a phase relation similar to the grids of the anodes of the groups 69., 6c and 59. The remaining groups of anodes, however, are energized so as to become conductive 30 electrical degrees later than they normally would under full load and this is accomplished by closing the switches corresponding to switches 27, 28, 29, 56 and 5t and the switches 22 and 13 and adjusting the phase shifters 20 and 54 so as to cause the anodes 6b, 6d, 6: and lea, we, lfie to become conductive during the latter third of the positive half cycle of anode voltage.

If, for instance, during half load operation after having the anodes 6e, 6e, 5e, H'Sb, lfid, lGi conduct it is desired to change to the anodes 6b, 6d, 6f, !62,, Hie, IE8, it is possible to make this commutation automatically by means of a synchronous switch operated by the synchronous motor 46. This switch mechanism may be arranged to energize relays which are positioned so as to operate in proper sequence the switches corresponding to switches 2!, 21. 28, 29 and 45. Inasmuch as such an arrangement is believed to be quite apparent to those skilled in the art, it has not been disclosed in Fig. la.

If it is desired to apply the grid control circuit disclosed in Fig. 1a to the rectifier arrangement disclosed in Fig. 5, it will be found from an inspection of the curves in Fig. 6 that at half load operation a certain sequence of excitation is he"- essary. Thus for instance, during one period the anodes 206b, 2i6c, 2|6d, 206:: are conductive then the anodes 2'60, 2I5d, 206e, 206: and later 2l6d, 206e, 265i, 2l6a, etc. From this it will be apparent that anode 206b, for instance, will not become conductive again until the next 720 10 electrical degree period. In order to obtain the proper grid excitation for this type of operation the following is the sequence of the switches operated in order to provide grid voltage: Switches 27, 28, 29 are closed then switch 2| and switches 45 and 43. Switch 43 energizes the alternating current generator 29 which preferably is of the type which produces rectangular wave shape of a frequency one-half that of the alternating current frequency of the line I. This introduction of an alternating current of one-half the frequency of line i and of substantially rectangular shape will so bias the grids of the rectifier during alternate cycles of the main line frequency so that, for instance, anode 2069. be comes conductive only once every 720 electrical degrees.

Fig. 4 represents another physical embodiment of the invention where the elementary groups are connected in series. The secondaries 5 and i5 of the transformer 3 supply, respectively, the anodes |fl6a|D6b|06z%|06d-|06el06f and ll6all6b-l|6c--l|6all6ell6r of two mercury vapor rectifiers, H0 and [26. The secondary 5 and the rectifier I Hi comprises one elementary group, while the secondary l5 and the rectifier I 20 comprises another elementary group. The two groups are connected in series so that the neutral point ll! of the secondary I5 is connected to the cathode III of rectifier H0 and the neutral point ID! of the secondary 5 and the cathode H! of the rectifier I25 are connected respectively to opposite sides of the line 2.

All the explanations which were given regarding the operation of the circuit represented in Fig. 1 apply to the operation of the circuit represented in Fig. 4, taking account of the fact that the instantaneous rectified voltage at the outer terminals of the converter is equal to the sum of both voltages that are induced in the phase windings connected to the two anodes which supply energy simultaneously, instead of being equal to their mean value. Likewise, the diagrams of Fig. 2 serve also to explain the operation of the circuit shown in Fig. 4 by assuming that the curves ab-cd-ej represent no longer the common voltages that are induced, respectively, in the phase windings (5a, l5a) (5b, I5b) (5c, l5c) (5d, l5a) (5e, l5e) (5r, I50, but twice these common voltages. The curves of Fig. 3 represent 0 likewise the trend of the variation of the theoretical phase displacement factor as a function of the value of the rectified voltage.

Fig. 5 represents another application of the invention to a twelve-anode rectifier, this example being particularly adapted to the generation of high-density currents in so far as the load current is equally distributed between four anodes which supply energy simultaneously.

In Fig. 5, the primary 204 of transformer 203 is connected to the network I. The two six-phase secondaries 205 and US are resolved, respectively, in two three-phase stars 205x and 2El5 on the one hand, and 2| 5X and 2|5y on the other hand. The neutral points 201x and 201 of the star connections 205x and 2ll5 are interconnected by means of an interphase coil 209. Likewise, the neutral points 2 l TX and 211;, of the stars 2l5X and 2| 5 are interconnected by means of the interphase coil 2 IS. The mid-points 2h"; and 224 of the coils 209 and 219 are interconnected by a third interphase coil 255, the midpoint 2&3 of which is connected to one side or" the line 2, the other side thereof being connected to the cathode 2H of the twelve-anode rectifier 21m.

The phase windings 205a, 205b, 2650, 285d, 225e, 295i on the one hand and 2i5a, 2l5b, 5c, 2l5a, 2l5e, 2l5r on the other hand which are enumerated in the order corresponding to the sense of direction of the phases, are connected, respec tively, to the anodes 2068., 206b, 2850, 25th, 22%, 206i and 2lEa, 2l6b, 2l'6c, 2i6d, 2|6e, 2|5r. As in the preceding case, the connections of the oontrol grid such as 2 l2 have not been represented.

The converter which is illustrated in Fig. 5 is resolved into four elementary groups which function in parallel and which are each constituted by the windings of one of the three-phase stars and the corresponding anodes, for instance by the star 205,; and the anodes 296a, 2e50, and 2056. As in the preceding explanations, it will be assumed that the transformer 293 has no leakage inductances and that the commutation between the anodes is instantaneous.

It will be easily seen that at every instant, four anodes, each corresponding to an elementary group, supply energy simultaneously and that there is still a possibility of four additional phase relations.

In Fig. 6, the curves at to represent the electromotive forces that are induced in the windings 205a or 2|5a,2fl5f or 2 l5r, which will be assumed to be identical to the windings 5a or |5a,-5f or I51 The first phase relation will be obtained by having the anodes supply energy naturally. The operation then is identical to that of two sixphase rectifiers which are provided with interphase coils that operate in parallel, the anodes 206a and 2l6a, for instance, supplying energy at the same time as the anodes 26Gb and 216s and becoming extinguished when the anodes 266C and 2l6c start.

In Fig. 6, the curves fa, ab,and e represent, as in the preceding explanations, the resultant voltage curves corresponding to the first phase relation. The diagram of the maximum rectified voltage is represented by the rippled curve A5o- B5oC5oD5o-E5oF5o; the theoretical phase displacement factor being equal to unity.

The second phase relation will be obtained by having only the anodes 20Er2il5a-2I8a-2l5b, and then the anodes 2i 6a2l6b26'6b2@6c6t. for instance, supply energy simultaneously. In Fig. 6, the curves faab, abbc, ejfa, represent the resultant voltage curves corresponding to the second phase relation. It will be easily seen that this second relation cannot correspond to an operation with a unity phase displacement factor. For instance, the changeover from the simultaneous output of anodes 2fl6r-f-E i E a 2l'8b, to the simultaneous output of the anodes 2|6a,-2l6b-206b-205c, cannot take place at the time 1550 corresponding to the point of intersection of curves faab and abbc, because the current supplied by the anode 206a could not be transferred to the anode 2950 as long as the voltage which is induced in the winding 265C is not at least equal to the voltage which is induced in the winding 2053., that is to say, prior to the time in corresponding to the point of intersection of curves a and c. The result is that the second phase relation does not permit obtaining a rectified voltage which exceeds that represented in Fig. 6 by the diagram A5uB51-B5a C51C5nD51--D50E51. It will be easily seen that the value of the rectified voltage obtained in this manner is equal to at least three quarters of the maximum voltage (not considering the voltage drop) and that the theoretical phase displacement factor is equal to phase relation.

This third phase relation permits the operation with a theoretical phase-displacement factor equal to unity. It will be seen that the changeover from the simultaneous output of anodes 236b2l6c2l6d20'6e (for instance) to the simultaneous output of anodes 2l6c2 I Eid206e 285i, necessitates the commutation of the current from the anode 20Gb to the anode 206:. This is possible at the time tsz, corresponding to the point 5 E51 where the curves bcde and ode intersect,

for the potential of the anode 2116f then becomes equal to and finally exceeds the voltage of anode 20%. The rectified-voltage diagram is then represented by the rippled line B51C51D51E51.

It will be seen that the rectified voltage then is equal to one half of the maximum rectified voltage.

The fourth possible phase relation corresponds to the simultaneous output of four anodes, two by two in phase opposition, that is to say at a rectified voltage equal to zero. With the first three phase relations, the functioning of the various anodes was such that they were equally loaded. With the fourth, only four anodes would be under load, and it is advantageous to establish the changes between the active and the inactive anodes. As in the case of the first example which has been described, the intermediate rectified voltage can be obtained by alternating the operation of the converter according to several phase relations. If, for instance, the anodes 206e2l6e2fl6f2!6f supply energy simultaneously, and at the time its the starting of anode 205a alone is permitted, the rectified-voltage diagram will follow the curve effa to the time L54, where the starting of the anode 206: will be permitted, and so forth. It will be thus possible to obtain the rectified-voltage diagram which is given by the line A52F53F54B52- A53A54-C52 which corresponds to a rectified voltage the value of which is between the maximum value and three quarters of the maximum value. Likewise, the explanations which have been already given make it possible to understand that a voltage diagram such as the one represented by the rippled curve D51C55 C56E51D55D5s-Fs1E55E5c-A51 can be obtained and permits obtaining values of rectified voltages that lie between three quarters and one half of the maximum value.

Contrary to what occurred in the first example, voltages less than one half the rectified voltage can be obtained by retarding the change-over from the output of a series of anodes to the output of the following series. For instance, the change-over from the simultaneous output of anodes 2fl6b2l6c2iiid2ii6e, to the simultaneous output of anodes 2|6c-2l6d2il6e2iir, requires simply that the current changes over from anode 20% to anode 206i. This commutation is possible, starting from the time i152 and may be retarded by any electrical degree which is at most equal to degrees; since anode is more positive than anode 26610 in the corresponding time interval, the commutation will be possible at the desired instant.

This method of regulation involving a time lag at the commutation, is the only one which can be used to obtain the rectified-voltage values comprised between one-half and one-fourth of the maximum voltage.

Voltages less than one-fourth of the maximum voltage can be obtained either by increasing the time lag at the commutation or by causing the two operations to alternate according to the phase relations corresponding to one-half of the rectified voltage and to zero rectified voltage. For instance, if the anodes 2G6b2ilic-?:ifid2ii-5e (curve bade) have a simultaneous output, and if at the time 1555, the anode 2l6r is released, the latter anode will extinguish the anode 286a, and the four anodes 2536b, Zlfic, 25569, Zlfir will have a simultaneous output which will give a. resultant voltage equal to zero. At the time. 1556, the anode 205d is released, it extinguishes anode rest, and the anodes 25%, 28%, 2l6e, Her will have a simultaneous output (representative curve cdef). After that one will change over to the simultaneous operation of anodes Zita, 298a, Zlfir, Etta, then 20%, 2l6e, ZlEr, 296a, and so forth, and ob.- tain thus voltage diagrams such as the one represented by the line F57-D58--D59A5'7-E58 E59 the points F57, D58 and A57 whose ordinate is zero coinciding on the abscissa axis with the representative points of the times tss, its and tss.

In Fig. '7, the line M7 M6 Z6 M5 Z5 shows the trend of the variation of the theoretical phase displacement factor corresponding to the circuit represented in Fig. 5, as a function of the ratio between the rectified voltage and the maximum rectified voltage Em.

It will be easy to follow the operation of this circuit as an inverter circuit, and it will be found that the representative curves of the theoretical phase displacement factor are still the same as those represented in Fig. '7.

The same results could be obtained by arranging the four elementary groups, which constitute the converter represented in Fig. 5, in series or in series parallel.

As in the preceding explanations the intermediate voltages can be also obtained and the phase displacement factor improved by alternating the. operations according to two different phase relations, at a frequency which is below the natural commutation frequency of the anodes.

Fig. 8 represents schematically one of the physical embodiments of the invention comprising an eighteen-anode rectifier. The supply transformer (the primary windings of which are not represented) comprises three secondary sixphase windings 395, 3l5 and 325, which are resolved respectively, in the three-phase stars, 305x and 30.5 3l5x and 3|5 325x and 325 the neutral points of the two three-phase stars comprising one of the secondaries, being interconnected by means of the single-phase coils 389, 3l9, and 329. The mid-points of these coils are interconnected by a three-phase coil 393, the neutral point N3 of which is connected to the negative pole of the network 2. The phase windings supply, respectively, the eighteen anodes (not shown) of the rectifier 3E0, the cathode 3H of which is connected to the positive side of the line 2. It will be noted that six anodes have a simultaneous output and the operation with a unity phase-displacement factor is possible for the rectified voltages that are equal to the maximum voltage in the case oi two thirds of the maximum voltage and of a zero rectified voltage. Other phase combinations are possible, which correspond to a theoretical phase displacement factor which is slightly less than unity. Particularly, one can verify that, for rectified voltage values between the maximum value and its two thirds, the theoretical phase displacement factor never goes below six hundredths of unity.

The examples of application which have been just described correspond to circuits for which the various possible phase combinations cor respond to the simultaneous output of the same number of anodes and for which the process which is the object of the invention is combined with the regulating process consisting in retarding asymmetrically the moments of ignition of the various anodes of the apparatus in question. The invention is in nowise limited to this particular embodiment. It is an easy matter to work out circuits by using the method constituting the object of the invention, in which the number of anodes that have a simultaneous output is not necessarily the same for all phase relations, and which do not require the asymmetrical loading of the anodes during one cycle for all the values of the continuous potential that differ from the maximum voltage.

Fig. 9 represents schematically a circuit in which two phase combinations correspond to the simultaneous output of two anodes and in which all the anodes have a simultaneous output during one cycle, a third combination of phases corresponding to the simultaneous output of three anodes.

The supply transformer (the primary windings of which are not shown) comprises two identical six phase secondaries 405 and M5, each one of which is resolved into three groups 495x, 405,, MHZ and 5X, 4I5 4152, each constituted by two windings in phase opposition. The secondary windings 465a, 405i and M51. 4|5r (which are enumerated in the order of the phase rotation) are connected, respectively, to the twelve anodes 406a, 406i and 416a Sr, of a. mercury vapor rectifier MG, which is provided with control grids (not shown) and the cathode 4 of which is connected to one side of the line 2.

The mid-points of the winding groups 405x and 5 4fl5 and 4|5z, 4052 and 5X, are interconnected, respectively, two by two, by means of windings 4o8xy, 408 z and 4082):, which are arranged, respectively, on the three legs of a threephase magnetic circuit comprising an interphase transformer or current divider 400. The midpoints of the three windings 408xy, 408 z and 408 are interconnected and are combined at the second pole of network 2.

If all the control grids permit the ignition of the anodes, two of the latter, for instance anodes 4|6r and 406a, will be compelled to have a simultaneous output or" equal currents, so that the direct-current ampere turns will be balanced in both halves of 308. When anode ilth becomes more positive than anode d l 8f, it has the tendency to start, but its current is limited to the magnetized current of winding ilitzx, and it can assume a considerable load only if one of the anodes corresponding to the winding group 405Z can start in its turn. Because of an inductance force in the winding 4582K, the potential difference between the anode lifia and the cathode manifests itself at the center point of the winding group 4852 and raises the potential of anodes 1535b and 486E. When the potential of anode 4223b is thus increased and attains the potential of anodes 4 if and 959. which are supplying energy, that anode will start in turn and will take the place of anode area, while at the same time anode 426;; will take the place of anode lltf, as a result of the electrornotive forces that are induced in the interphase transformer This commutation will be possible as soon as the sum of the voltages that are induced in the windings 4 i 5a and 36513 is equal to the sum of the voltages induced in windings 495i and ilia, that is to say when the voltages induced in windings and 3051; will be equal.

The operation which has been just described corresponds to the maximum rectified voltage. The eifect of the control grids makes it possible to retard the commutation and to decrease the value of the rectified Voltage.

A second phase relation can be obtained, which makes possible a phase displacement factor equal to unity, by having only two anodes supply energy simultaneously, which correspond, one to the secondary 405 and the other to the secondary H5, and which are fed by the windings the voltages of which are one-third of a cycle out of phase, for instance the anodes 495a and 516c. With the foregoing explanations it is easy to see that if the anodes 305b and 4454 are released, all the other anodes being blocked, they will assume the load and will extinguish the two anodes imit and iiiic when the sum of the voltages of the phase windings 435i, and Alfie will be equal to the sum of the voltages of windings 495a and Q5 5c. As in the preceding, the rectified voltage can be lowered by retarding their commutation.

The first two phase relations make it possible to obtain, with a theoretical phase displacement factor equal to unity, rectified voltages whose values are in the ratio of It will be noted that for the two operations which have been just described, each anode supplies energy for one-sixth of a cycle during each cycle.

A third phase relation is obtained by allowing only the anodes corresponding to one or" the two secondaries to supply energy, for instance, the anodes 466a, 496i. balance of the direct-current ampere turns on the interphase transformer 409, can only be obtained if three anodes each corresponding to one of the groups 405x, 4l5 and diifiz have a simultaneous output. If the ignition between anodes is not retarded, the value of the rectified voltage is equal to the maximum voltage corresponding to the first combination of phases multiplied by and the theoretical displacement factor is equal to unity.

As in the cases that have been described in the preceding explanations, it is easy to bring about a permutation of the operations according to two phase relations, in order to obtain intermediate voltage Values.

The invention is applicable to inverters or to static frequency changers. Fig. 10 shows a physical embodiment, in conformity with the invention, of an inverter which insures the transfer of energy from a three-phase network to a singlephase network of lower frequency. In Fig. 10, two converters 553i? and ME), identical to the one represented in Fig. l, are interconnected accord ing to the well-known figure eight circuit. Cathode i i for instance, being connected to the neutral point N3, of the interphase coil 668 of the other converter, and the single-phase network 582 being connected-to the two cathodes 5i i and 6! l of both converters.

According to the well-known technique, each converter supplies a half-wave of the low-frequency voltage. The diagrams of Fig. 11 reproduce the curves a f, ab fa, ac ib, already represented in Fig. 2.

The explanations which have been given in connection with thc'circuit shown in Fig. 1 make it obvious that the successive combinations of the simultaneous output of the anodes connected to windings 505a, H50, Eflii and 5l5c, 505 and 5|5e, 595s and 5l5e, 5'35f and 555e, 5i35r and 5l5a, 5050 and 5min make it possible to obtain a voltage diagram A100B100-C100D1c0E1u0Fi0oG1u0 H100I10o; which represent the half-wave furnished by the converter 5%, of a single-phase voltage the frequency of which is equal to onethird of the supply frequency, the converter 69E? supplying the second half wave in the same manner.

Similarly, one can obtain the voltage of the half frequency of which one-half wave is represented by the rippled curve Jo-B10oC1occonverter 5% and set can likewise (as has been already explained in connection with Fig. 1) operate as an inverter, it is evident that the network 5ii2 will be able to return energy into network 1 whenever necessary, and that consequently, according to a well-known practice, the converter represented in Fig. 10 will be able to supply an alternating current which is out of phase with respect to the network voltage 502, that is to say, will be able to supply or absorb reactive power.

The physical embodiments which have been just described may be considered as comprising a part of the invention, but they are not a limitation of the invention. Numerous variations may be easily conceived which will still remain within the scope of the invention, particularly that-single anode valves may be used, since the secondary windings of the transformers can be polygon-connected as well as star-connected.

While I have shown the application of my invention to certain specific embodiments it will, of

course, be understood that I do not wish to be limited thereto, since it is apparent that the principles herein described are susceptible of numerous other applications without departing from the spirit and scope of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In an electric valve converting system including a plurality of groups of valves each provided with a polyphase winding, and an inductive winding interconnecting said polyphase windings, the method of operation which comprises maintaining certain of said valves in each group non-conductive to obtain a reduced power output,

and at regular intervals alternating the function of said non-conductive valves with said conductive valves.

2. In an electric valve converting system having a plurality of polyphase windings interconnected by an inductive winding, and an electric valve having a plurality of controlled anodes connected to said polyphase windings, the methodof reducing the output of said system which comprises rendering inactive certain anodes associated with each of said polyphase windings and retarding the moment of ignition of the remaining anodes thereby to improve the power factor of said system at said r duced power output. 7

In an electric valve converting system having a plurality of polyphase windings interconnected by an inductive winding, and a group of controlled electric valves for each of said polyphase windings, the method of reducing the power output thereof which comprises gradually retarding the time of ignition of said valves up to a certain point in the load characteristicof said system, then rendering inactive certain valves of each group and again gradually retarding the time of ignition of said valves thereby to improve the theoretical phase displacement factor for said operating range.

l. In an electric valve converting system having a plurali y of polyphase windings interconnected by an inductive winding, and a group of controlled electric valves for each of said polyphase windings, the method of controlling the poweroutput thereof which comprises rendering inactive certain Valves in each group while permitting the remaining valves to become active for a certain time interval and during a subsequent time interval rendering inactive the valves which were active in the preceding interval while permitting said first mentioned certain valves to become active. 7

5. In an electric valve converting system including a plurality of polyphase windings interconnected by an inductive winding, and a group of controlled electric valves for each of said pclyphase windings, the method of operation which comprises maintaining non-conductive certain valves of each group to obtain a certain reduced power output with a unity power factor and controlling said valves in the operating region between said certain reduced power output and full power output by permuting between the operating sequences of said valves at said different power outputs and simultaneously varying the ti e of ignition of said valves thereby to improve the po\ er-factor load-characteristic of said system.

JEAN AUGIER.

ilO 

